Transient Spectroscopy Data Research Articles

Competing Nonadiabatic Relaxation Pathways for Near-UV Excited ortho-Nitrophenol in Aqueous Solution.

Nitrophenols are atmospheric pollutants found in brown carbon aerosols produced by biomass burning. Absorption of solar radiation by these nitrophenols contributes to atmospheric radiative forcing, but quantifying this climate impact requires better understanding of their photochemical pathways. Here, the photochemistry of near-UV (λ = 350 nm) excited ortho-nitrophenol in aqueous solution is investigated using transient absorption spectroscopy and time-resolved infrared spectroscopy over the fs to μs time scale to characterize the excited states, intermediates, and photoproducts. Interpretation of the transient spectroscopy data is supported by quantum chemical calculations using linear-response time-dependent density functional theory (LR-TDDFT). Our results indicate efficient nonradiative decay via an S1(ππ*)/S0 conical intersection leading to hot ground state ortho-nitrophenol which vibrationally cools in solution. A previously unreported minor pathway involves intersystem crossing near an S1(nπ*) minimum, with decay of the resulting triplet ortho-nitrophenol facilitated by deprotonation. These efficient relaxation pathways account for the low quantum yields of photodegradation.

Femtosecond Infrared Spectroscopy Resolving the Multiplicity of High-Spin Crossover States in Transition Metal Iron Complexes.

Tuning the photophysical properties of iron-based transition-metal complexes is crucial for their employment as photosensitizers in solar energy conversion. For the optimization of these new complexes, a detailed understanding of the excited-state deactivation paths is necessary. Here, we report femtosecond transient mid-IR spectroscopy data on a recently developed octahedral ligand-field enhancing [Fe(dqp)2]2+ (C1) complex with dqp = 2,6-diquinolylpyridine and prototypical [Fe(bpy)3]2+ (C0). By combining mid-IR spectroscopy with quantum chemical DFT calculations, we propose a method for disentangling the 5Q1 and 3T1 multiplicities of the long-lived metal-centered (MC) states, applicable to a variety of metal-organic iron complexes. Our results for C0 align well with the established assignment toward the 5Q1, validating our approach. For C1, we find that deactivation of the initially excited metal-to-ligand charge-transfer state leads to a population of a long-lived MC 5Q1 state. Analysis of transient changes in the mid-IR shows an ultrafast sub 200 fs rearrangement of ligand geometry for both complexes, accompanying the MLCT → MC deactivation. This confirms that the flexibility in the ligand sphere supports the stabilization of high spin states and plays a crucial role in the MLCT lifetime of metal-organic iron complexes.

Framework-Topology-Controlled Singlet Fission in Metal-Organic Frameworks.

Singlet fission (SF) has been explored as a viable route to improve photovoltaic performance by producing more excitons. Efficient SF is achieved through a high degree of interchromophoric coupling that facilitates electron superexchange to generate triplet pairs. However, strongly coupled chromophores often form excimers that can serve as an SF intermediate or a low-energy trap site. The succeeding decoherence process, however, requires an optimum electronic coupling to facilitate the isolation of triplet production from the initially prepared correlated triplet pair. Conformational flexibility and dielectric modulation can provide a means to tune the SF mechanism and efficiency by modulating the interchromophoric electronic interaction. Such a strategy cannot be easily adopted in densely stacked traditional organic solids. Here, we show that the assembly of the SF-active chromophores around well-defined pores of solution-stable metal-organic frameworks (MOFs) can be a great platform for a modular SF process. A series of three new MOFs, built out from 9,10-bis(ethynylenephenyl)anthracene-derived struts, show a topology-defined packing density and conformational flexibility of the anthracene core to dictate the SF mechanism. Various steady-state and transient spectroscopic data suggest that the initially prepared singlet population can prefer either an excimer-mediated SF or a direct SF (both through a virtual charge-transfer (CT) state). These solution-stable frameworks offer the tunability of the dielectric environment to facilitate the SF process by stabilizing the CT state. Given that MOFs are a great platform for various photophysical and photochemical developments, generating a large population of long-lived triplets can expand their utilities in various photon energy conversion schemes.

Unraveling coupled folding and binding dynamics of ribonuclease S with transient infrared spectroscopy

Molecular interactions of proteins consisting of coupled disorder-to-order transitions and spontaneous binding have a crucial role in understanding their biological function and working mechanism. Here we investigate a well-known globular binding system, ribonuclease S (RNase S), to study the dynamics of protein-peptide interaction. Various biophysical approaches have investigated the binding mechanism of two proteolytic fragments, S-peptide and S-protein, which non-covalently bind to form an active enzyme RNase S. Due to challenges in dissecting the coupled nature of protein binding and folding, molecular level details remain elusive. Here, we use amide-I vibrations as a label-free probe of protein secondary structure and temperature as a control variable. By temperature-jump two-dimensional infrared spectroscopy (t-2DIR), we track the thermally-induced conformational changes associated with relaxation time that span from nanoseconds to many minute timescales. Additionally, we compare the results of an unlabeled complex with one containing a 13C=18O isotope-label of a residue buried within the helical segment of S-peptide. t‑2DIR measurements of labeled- and unlabeled- RNase S and isolated S-protein reveal two kinetic components on millisecond (fast) and second (slow) time scales. Both slow and fast phases are dominated by β-sheet disordering signatures that indicate S-protein unfolding. The isotope-label exhibits significant amplitude in the slow phase, suggesting this time window corresponds to both unbinding and unfolding of the S-peptide. Together, these results indicate that thermally-induced partial unfolding of the S-protein destabilizes the hydrophobic binding pocket containing S-peptide that, in turn, promotes unbinding and unfolding of the S-peptide. Our direct observations from transient spectroscopic data demonstrate the role of a conformational transition state during assembly of the RNase S complex.

KiMoPack: A python Package for Kinetic Modeling of the Chemical Mechanism.

Herein, we present KiMoPack, an analysis tool for the kinetic modeling of transient spectroscopic data. KiMoPack enables a state-of-the-art analysis routine including data preprocessing and standard fitting (global analysis), as well as fitting of complex (target) kinetic models, interactive viewing of (fit) results, and multiexperiment analysis via user accessible functions and a graphical user interface (GUI) enhanced interface. To facilitate its use, this paper guides the user through typical operations covering a wide range of analysis tasks, establishes a typical workflow and is bridging the gap between ease of use for less experienced users and introducing the advanced interfaces for experienced users. KiMoPack is open source and provides a comprehensive front-end for preprocessing, fitting and plotting of 2-dimensional data that simplifies the access to a powerful python-based data-processing system and forms the foundation for a well documented, reliable, and reproducible data analysis.

Enhanced luminescence efficiency and thermal stability via introduction of non-rare earth Bi3+ in Gd5Si2BO13:Eu3+

A series of Gd5Si2BO13:Eu3+ and non-rare earth Bi3+ ions doped Gd5Si2BO13:Eu3+ phosphors was successfully synthesized via high-temperature solid-state method, and the as-obtained phosphors were studied on their phase structures, luminescence characteristics, thermal stability and luminescence lifetime. Transient fluorescence spectroscopy data show that the addition of Bi3+ can obviously enhance the emission intensity of Eu3+ in the near-ultraviolet band owing to energy transfer from Bi3+ to Eu3+. Besides, the addition of Bi3+ can improve the thermal stability of a single-doped phosphor Gd5Si2BO13:0.35Eu3+ from 70.33% to 87.45% at 150 °C and the quantum yield from 58.80% to 78.61% at room temperature. Finally, Gd5Si2BO13:Eu3+,Bi3+ was used to encapsulate white light-emitting diodes (WLEDs) with green ((Ba,Sr)2SiO4:Eu2+) and blue (BaMgAl10O17:Eu2+) commercial phosphors. The color rendering index of WLEDs was calculated to be larger than 90, and the color temperature was estimated to be 4300–4500 K, which demonstrate that Gd5Si2BO13:Eu3+,Bi3+ can be regarded as a red phosphor with great potential application. This paper can provide a new insight into design of high-efficiency phosphors by introducing non-rare earth Bi3+ ions via energy transfer from Bi3+ to Eu3+.

On the intersystem crossing rate in a Platinum(II) donor-bridge-acceptor triad.

The rates of ultrafast intersystem crossing in acceptor-bridge-donor molecules centered on Pt(II) acetylides are investigated. Specifically, a Pt(II) trans-acetylide triad NAP--Pt--Ph-CH2-PTZ [1], with acceptor 4-ethynyl-N-octyl-1,8-naphthalimide (NAP) and donor phenothiazine (PTZ), is examined in detail. We have previously shown that optical excitation in [1] leads to a manifold of singlet charge-transfer states, S*, which evolve via a triplet charge-transfer manifold into a triplet state 3NAP centered on the acceptor ligand and partly to a charge-separated state 3CSS (NAP--Pt-PTZ+). A complex cascade of electron transfer processes was observed, but intersystem crossing (ISC) rates were not explicitly resolved due to lack of spin selectivity of most ultrafast spectroscopies. Here we revisit the question of ISC with a combination and complementary analysis of (i) transient absorption, (ii) ultrafast broadband fluorescence upconversion, FLUP, which is only sensitive to emissive states, and (iii) femtosecond stimulated Raman spectroscopy, FSR. Raman resonance conditions allow us to observe S* and 3NAP exclusively by FSR, through vibrations which are pertinent only to these two states. This combination of methods enabled us to extract the intersystem crossing rates that were not previously accessible. Multiple timescales (1.6 ps to ∼20 ps) are associated with the rise of triplet species, which can now be assigned conclusively to multiple ISC pathways from a manifold of hot charge-transfer singlet states. The analysis is consistent with previous transient infrared spectroscopy data. A similar rate of ISC, up to 20 ps, is observed in the trans-acetylide NAP--Pt--Ph [2] which maintains two acetylide groups across the platinum center but lacks a donor unit, whilst removal of one acetylide group in mono-acetylide NAP--Pt-Cl [3] leads to >10-fold deceleration of the intersystem crossing process. Our work provides insight on the intersystem crossing dynamics of the organo-metallic complexes, and identifies a general method based on complementary ultrafast spectroscopies to disentangle complex spin, electronic and vibrational processes following photoexcitation.

How to scrutinize adsorbed intermediates observed by in situ spectroscopy: Analysis of Coverage Transients (ACT)

The understanding of catalytic mechanisms is enhanced by the observation of surface intermediates at reaction conditions using spectroscopic techniques, but this is insufficient, as the observed species may not be involved in the reaction. This work describes a general method of analysis of hydrogenation or oxidation reactions which uses transient spectroscopic data to determine whether an adsorbed species is a reactive intermediate or a spectator on the surface. The assumptions and limitations of the method are summarized. Although the technique is approximate, it is easy to implement, and provides order-of-magnitude estimates of the rate of reaction of an intermediate. The method consists of measuring the change of coverage of the species with time, dθ/dt, during adsorption in inert gas or at reaction conditions. An example is given with the hydrodeoxygenation of the model compound γ-valerolactone (GVL) using a Ni2P/MCM-41 catalyst, one of the most effective catalysts reported for the transformation. The reaction is relevant to the upgrading of bio-oil derived from pyrolysis of lignocellulosic feedstocks. The kinetics of the reaction and observation by in situ infrared spectroscopy of adsorbed GVL and its transformation to pentanoic acid are consistent with a Langmuir-Hinshelwood mechanism. Analysis by in situ transient X-ray absorption fine structure shows that the adsorbed GVL is a true reaction intermediate.

Digestive Ripening-Mediated Growth of NaYbF4:Tm@NaYF4 Core–Shell Nanoparticles for Bioimaging

Size and size distribution control along with the excitation/emission wavelength manipulation are the most indispensable for both fundamental research and applications of upconverting nanophosphors. In contradiction to the usual Ostwald ripening process, digestive ripening-mediated growth of core–shell nanostructures is observed in a lanthanide-doped ternary fluoride system. Time-dependent size evolution of NaYbF4:Tm@NaYF4 nanoparticles is systematically investigated, and a Ksp-involved growth mechanism is proposed to better understand this unusual phenomenon. The upconversion luminescence (UCL) intensity and branch ratio of near-infrared to visible emission (NIR/VIS) dramatically increase due to the NaYF4 coating but subsequently undergo a decline when an additional SiO2 layer grows outside. As a comparison, an alternant type of core–shell–shell nanostructure, NaYbF4:Tm@SiO2@NaYF4, is further constructed, in which both the UCL intensity and NIR/VIS ratio exhibit an inverse trend. The comparative analysis on the basis of steady and transient spectroscopic data from NaYbF4:Tm, and alternatively coated by silica and NaYF4 nanoshells, respectively, demonstrates that the UCL properties can be regulated through core–shell engineering once the optical behavior of the surface states can be manipulated. The results provide a promising alternative strategy to fabricate a uniform core–shell nanostructure with multicompositions, and this NIRin–NIRout luminescent profile deserves an expectable vision for the subsequent clinical applications of UCL imaging for deep biological tissues.

PrFeO3 Photocathodes Prepared Through Spray Pyrolysis

AbstractPerovskite oxides are receiving wide interest for photocatalytic and photoelectrochemical devices, owing to their suitable band gaps for solar light absorption and stability in aqueous applications. Herein, we assess the activity of PrFeO3 photocathodes prepared by using spray pyrolysis and calcination temperatures between 500 and 700 °C. Scanning electron microscopy shows corrugated films of high surface coverage on the conductive glass substrate. The electrochemically active surface area shows slight decreases with temperature increases from 500 to 600 and 700 °C. However, transient photocurrent responses and impedance spectroscopy data showed that films calcined at higher temperatures reduced the probabilities of recombination due to trap states, resulting in faster rates of charge extraction. In this trade‐off, a calcination temperature of 600 °C provided a maximum photocurrent of ‐130±4 μA cm−2 at +0.43 VRHE under simulated sunlight, with an incident photon‐to‐current conversion efficiency of 6.6 % at +0.61 VRHE and 350 nm and an onset potential of +1.4 VRHE for cathodic photocurrent.

Upconversion luminescent perovskite CaTiO3:Yb3+/Er3+ nanocubes

Perovskite CaTiO3:Yb3+,Er3+ nanocubes with tunable upconversion luminescence (UCL) were prepared by a solvothermal method. The structure and morphology of the final product after being sintered at 700 °C for 2 h were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The upconversion emissions of CaTiO3:Yb3+,Er3+ under the excitation of 980 nm diodes laser consist of two typical emission bands from 4F9/2 → 4I15/2 and 4S3/2, 2H11/2 → 4I15/2 transitions. The UCL properties depending on the doping contents of Yb3+ to Er3+ were investigated in detail. The maximum R/G ratio was observed at Yb3+ concentration of 15 mol%. The multiphotonic upconversion mechanism of controllable red-to-green ratio in CaTiO3:Yb3+,Er3+ nanocubes was addressed based on both steady state and transient spectroscopy data.

Deep-level transient spectroscopy characterization of electrical traps in p-type multicrystalline silicon with gettering and hydrogenation process

In p-type multicrystalline silicon solar cells, defects in silicon bulk are well known to act as carrier recombination centers and considerably affect the conversion efficiency of cells. In this study, effective minority carrier lifetime (τeff) mapping and deep-level transient spectroscopy (DLTS) data were utilized to evaluate gettering efficiency and hydrogenation passivation in bulk defects. Microwave photoconductivity decay method was used to illustrate the effect of phosphorus gettering behavior and hydrogenation process on the bulk carrier lifetime of multicrystalline silicon. Results showed that τeff drastically improved after gettering and further increased after hydrogenation. DLTS results further revealed six traps corresponding to interstitial iron (Fei), Fe–O complex, FeB complex, vanadium, Fei complex and divacancy oxygen. The main trap in a nongettered sample was related to Fei, with two deep levels at about EV + 0.373 and EV + 0.470 eV. However, the Fei complex was the main trap in the gettered sample, with two deep traps at about EV+0.390 and EV+0.442 eV. Finally, we concluded that Fei defect and FeB complex can be significantly removed by phosphorus diffusion gettering. Moreover, divacancy-oxygen complexes, vanadium and Fe–O defects were effectively passivated by hydrogenation process. Fei complex, however, was hardly removed by these two process.

Semiconductor steady state defect effective Fermi level and deep level transient spectroscopy depth profiling

The widely used deep level transient spectroscopy (DLTS) theory and data analysis usually assume that the defect level distribution is uniform through the depth of the depletion region of the n—p junction. In this work we introduce the concept of effective Fermi level of the steady state of semiconductor, by using which deep level transient spectroscopy depth profiling (DLTSDP) is proposed. Based on the relationship of its transition free energy level (TFEL) and the effective Fermi level, the rules of detectivity of the defect levels are listed. Computer simulation of DLTSDP is presented and compared with experimental data. The experimental DLTS data are compared with what the DLTSDP selection rules predicted. The agreement is satisfactory.

Deep-level sensitization in Ge-SiO2 composite films observed by photocurrent spectroscopy

We studied the temperature dependence of the photocurrent spectra of a Ge-SiO2 composite thin film. We found that the spectral position of the photocurrent peak is determined by the competition between absorption and non-radiative recombination and that its temperature dependence is associated with the population variation of the energetically deep levels in the system under “thermal quenching” conditions. Combining these results with our previous deep-level transient spectroscopy data enables the association of these levels with the quantum confinement effect. We thus identify here a non-radiative recombination process associated with deep-level sensitization that stems from quantum confinement.

Annealing effects and DLTS study on PNP silicon bipolar junction transistors irradiated by 20 MeV Br ions

Isochronal anneal sequences have been carried out on 3CG130 silicon PNP bipolar junction transistors (BJTs) irradiated with 20MeV bromine (Br) heavy ions. The Gummel curve was utilized to characterize the annealing behavior of defects in both the emitter-base depletion region and the neutral base. The results show that the base current (IB) decreases with the increasing annealing temperature, while the collector current (IC) keeps invariably. The current gain varies slightly, when the annealing temperature (TA) is lower than 500K, while varies rapidly at TA>550K, and the current gain of the 3CG130 BJT annealing at 700K almost restore to that of the pre-radiation transistor. The deep level transient spectroscopy (DLTS) data was used to assign the relative magnitude of each of the important defects. Based on the in situ electrical measurement and DLTS spectra, it is clear that the V2(+/0) trap is the main contribution to the degradation of current gain after the 20MeV Br ions irradiation. The V2(+/0) peak has many characteristics expected for the current gain degradation.

Annealing effects and DLTS study on NPN silicon bipolar junction transistors irradiated by heavy ions

Isochronal anneal sequences have been carried out on 3DG112 silicon NPN bipolar junction transistors (BJTs) irradiated with 20MeV bromine (Br) heavy ions. The Gummel curve is utilized to characterize the annealing behavior of defects in both the emitter-base depletion region and the neutral base. We find that the base current (IB) decreases with the increasing annealing temperature, while the collector current (IC) remains invariable. The current gain varies slightly, when the annealing temperature (TA) is lower than 400K, while varies rapidly at TA<450K, and the current gain of the 3DG112 BJT annealing at 700K almost restore to that of the pre-radiation transistor. Deep level transient spectroscopy (DLTS) data is used to assign the relative magnitude of each of the important defects. Based on the in situ electrical measurement and DLTS spectra, it is clear that the V2(−/0)+V-P traps are the main contribution to the degradation of current gain after the 20MeV Br ions irradiation. The V2(−/0)+V-P peak has many of the characteristics expected for the current gain degradation.

Simulating the Radiation Response of GaAs Solar Cells Using a Defect-Based TCAD Model

Three-dimensional TCAD simulations are used for physics-based prediction of space radiation effects in III-V solar cells, and compared with experimentally measured characteristics of a p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> n GaAs solar cell with AlGaAs window. Dark and illuminated I-V curves as well as corresponding performance parameters are computed and compared with experimental data for 2 MeV proton irradiation at various fluences. We analyze the role of majority and minority carrier traps in the solar cell performance degradation. The traps/defects parameters used in the simulations, for n-type and p-type GaAs, are derived from Deep Level Transient Spectroscopy (DLTS) data. The physics-based models allow a good match between simulation results and experimental data. However, assuming the device performance is dominated by a single recombination center is not adequate to completely capture the degradation mechanisms controlling the photovoltaic performance.

Optical identification and electronic configuration of tungsten in 4H- and 6H-SiC

Several optically observed deep level defects in SiC are still unidentified and little is published on their behavior. One of the commonly observed deep level defects in semi-insulating SiC is UD-1.This report suggests that UD-1 is Tungsten related, based on a doping study and previously reported deep level transient spectroscopy data, as well as photo-induced absorption measurements. The electronic levels involved in the optical transitions of UD-1 are also deduced. The transitions observed in the photoluminescence of UD-1 are from a Γ4C3v, to two different final states, which transform according to Γ5C3v⊕Γ6C3v and Γ4C3v, respectively.

ChemInform Abstract: Prototype Supported Metal Cluster Catalysts: Ir4 and Ir6

AbstractReview: 66 refs.

Prototype Supported Metal Cluster Catalysts: Ir4 and Ir6

AbstractThe following is a review of the synthesis, chemistry, and catalytic properties of small, well‐defined supported metal clusters, Ir4 and Ir6. These supported clusters are synthesized either by adsorption of ligated molecular iridium clusters with tetrahedral and octahedral frameworks, for Ir4 and Ir6, respectively, or by adsorption of single‐iridium‐atom complexes followed by treatment to form clusters on the supports. The most insightful characterizations of supported metal clusters have been carried out by infrared and X‐ray absorption spectroscopies and aberration‐corrected scanning transmission electron microscopy. Reactions of the supported clusters include ligand modifications and ligand removal, oxidative fragmentation, and migration leading to metal aggregation. Reactivities and catalytic properties of the clusters (e.g., for olefin hydrogenation) depend on the cluster size and the support (which acts as a ligand) and are distinct from those of supported particles that resemble bulk iridium. Transient spectroscopic data illustrate that the structures of the active species in ethylene hydrogenation catalysis switch reversibly from Ir4 to single‐iridium‐atom complexes in response to changes in the ethylene/H2 ratio in the gas phase. Density functional theory has been used to predict properties of these clusters, notably their ability to add hydrogen, and the agreement between theory and experiment is good. Recent advances in the characterization of supported Ir4 and Ir6 clusters have been crucial in the elucidation of their behavior, and these methods apply as well to the more complicated structures of industrial supported metal catalysts.